This invention relates to the field of variable input-resistance power supply circuits, and in particular, to the use of such power supply circuits in temperature control systems.
The most basic control circuit for furnaces and air conditioners is shown in FIG. 1A. A secondary 1 of a transformer provides a source of power. It is connected to the coil 2 of an activation relay and to a bimetallic switch 3. The relay and the transformer are usually located near the furnaces or air conditioners and the bimetallic switch 3 is part of a thermostat located in the area in which temperature control is desired.
When the temperature reaches a preset value in the system in FIG. 1A, the bimetallic switch closes causing current to flow through the coil of the activation relay. That coil current in turn causes the relay contacts to turn on or off the heating or air conditioning system to which the contacts are connected.
When the temperature changes from that preset value in the proper direction, the bimetallic switch opens. This stops the current flow through coil 2 and causes the heating or air conditioning system to which the relay contacts are connected to change state.
Recently, there have appeared on the market sophisticated thermostats which provide a wide range of capabilities for temperature control. Many of these thermostats allow the person using them to program temperature settings for an entire day. Unfortunately, many of these thermostats are incompatible with existing temperature control systems.
Existing temperature control systems commonly contain circuits similar to the one in FIG. 1A. One way of connecting the new thermostats to the existing circuits is to replace the bimetallic switch in FIG. 1A by a relay controlled by a sophisticated thermostat. There remains, however, the question of how to power that thermostat and these new thermostats typically have a large number of logic elements and may even contain a microprocessor.
One new thermostat uses a control relay in place of a bimetallic switch. The supply voltage for the thermostat is the voltage across the control relay when that relay is open. When the relay closes to activate the system, the thermostat switches to battery power.
The use of a battery, however, makes a thermostat unnecessarily large and expensive. Another problem is that most batteries cannot power a thermostat constructed from the most common, inexpensive logic elements. A thermostat using battery power may have to use low-power components, typically of the CMOS variety, which are more expensive than bipolar or NMOS components.
Another alternative is to power the thermostat from a power source entirely separate from the heating or air conditioning units. This alternative has an added cost of connecting external power. A more troublesome problem with the use of external power occurs if the external power to the thermostat fails, for example because of a blown fuse. Power will still be supplied to essentially uncontrolled heating and cooling systems.